Pressure sensors are used to measure pressures in a number of industrial applications. One type of pressure sensor is known as a deflectable-diaphragm pressure sensor. In these pressure sensors, a pressure is applied, either directly or through an incompressible fill fluid, to a deflectable diaphragm. An electrode, such as a capacitive plate, is coupled to the deflectable diaphragm such that movement of the diaphragm plate changes an electrical characteristic, such as capacitance, which can be measured and related to the pressure. Deflectable-diaphragm based pressure sensors are used in differential, absolute, and gage pressure sensors.
Pressure sensors frequently operate in harsh environments such as high process temperatures or corrosive process fluids. One technique which has been used under such harsh conditions is to separate and isolate the pressure sensor from the process fluid whose pressure is being measured. One technique uses an isolation diaphragm which has process fluid on one side and an isolation fluid is on the other side. The isolation fluid couples the isolation diaphragm to the sensing diaphragm of the pressure sensor. As the process fluid pressure changes, the isolation diaphragm responsively deflects which causes the change in pressure to be transferred through the isolation fluid to the pressure sensor diaphragm. Pressure sensors with isolation diaphragms are well known in the art. However, to create such an isolated system adds cost and complexity. Further, the fluid can leak due to over pressure or mechanical damage resulting in loss of fluid volume or fill fluid which has been contaminated by process fluid.
In some applications, it is desirable to use a single-crystal pressure sensor. Single-crystal pressure sensors offer a number of advantages over other types of deflectable-diaphragm pressure sensors. The single-crystal pressure sensor provides extremely low hysteresis, high frequency response, and extreme resistance to both chemical attack and thermal breakdown. One particular single-crystal pressure sensor is constructed from sapphire. Since the single-crystal pressure sensor has such extreme resistance to chemical attack and thermal breakdown, it can be used in applications where no fill fluid is used. Thus, the process fluid may directly contact the single-crystal pressure sensor diaphragm. Accordingly, for some applications, single-crystal pressure sensors are a primary choice for process fluid pressure measurement.
While the process control measurement industry has developed significant diagnostic capabilities for determining the health and operational status of process communication, process device electronics, and even some process sensors, such diagnostics for single-crystal pressure sensors have not been developed. U.S. Pat. No. 7,918,134 provides a thermal-based diagnostic system for a pressure transmitter where a heat pulse is applied to a fill fluid in a process fluid pressure transmitter. The heat pulse affects the volume of the fill fluid and the changing volume can be measured with the pressure sensor in order to diagnose and/or verify operation of the pressure sensor. While such diagnostics are highly useful for fill fluid-based process fluid pressure sensor systems, they do not address applications where no such fill fluid is provided. Specifically, in some single-crystal pressure sensor applications, the process fluid itself is applied directly to the deflectable diaphragm of the pressure sensor. Accordingly, there is no fill fluid, and thus the diagnostics of the '134 patent are not usable.
Providing enhanced diagnostics and verification relative to single-crystal pressure sensors would increase the reliability and enhance field maintenance relative to applications where such sensors are employed.